Research of preparation and image processing method of PDMS micropost arrays with opaque top surfaces

Cells use force as a mechanical signal to sense and respond to their microenvironment. To this end, PDMS micropost arrays has been widely used in cell traction force measurement, which allows calculating magnitude and direction of cell force from the deflection of the micropost. However, the inherent transparency of PDMS leads to the complexity of image processing algorithm when retrieving the top surfaces centroids of the deflected microposts. A method of modifying the top surfaces of PDMS microposts using magnetic beads was proposed to attain opaque top surface of the micropost, enabling a higher contrast between the top surface and the substrate of the micropost array. Magnetic beads were brought into the molds by applying a magnetic field and casted in the PDMS microposts during soft-lithography process. The modified micropost arrays with opaque top surfaces were imaged using an inverted microscope as disk-shaped patterns top surface. The centroids of disk-shaped patterns in the microscopic image were calculated using regionprops function, while Hough transform was required to process the ring-shaped patterns of the conventional transparent microposts made with pure PDMS. The experimental results show that the contrast between the top surface and the substrate is significantly improved, hence precluding the need of using Hough transform, reducing the algorithm complexity in image processing, and improving the precision of micropost positioning.